Digital radiographic imaging systems, such as those using photostimulable storage phosphor (PSP), amorphous selenium, amorphous silicon, charge-coupled device (CCD), and metal oxide semiconductor-field effect transistor (MOSFET) technology, can produce adequate image quality over a much broader range of exposure levels than that of screen/film imaging systems. In screen/film imaging, the final image brightness and contrast are indicative of over- and underexposure. In digital imaging, brightness and contrast are often determined entirely by digital post-processing of the acquired image data. Over- and underexposures are not readily recognizable.
In 2008, the International Electrotechnical Commission (IEC) developed and published International Standard IEC 62494-1, “Exposure Index of Digital X-Ray Imaging Systems”, on the definition and scaling of the exposure index for digital radiography. According to the standard, the EI shall be proportional to the exposure (air kerma) and shall be scaled as EI=100*X, where X is the air kerma at the detector, at the calibration beam quality. The EI allows the operator to judge if an image was taken at a detector exposure level suitable for the intended level of image quality.
There is a significant degree of variability in the selection of exposure techniques to control X-ray exposure levels received by a patient undergoing an X-ray for diagnosis. Typically, the technologist visually assesses the thickness of the patient before making the selections. The combination of the patient thickness, the type of imaging receptor (e.g., computed radiography (CR), or digital radiography (DR)), and the choice of exposure techniques directly influences both the noise appearance and contrast in the captured image.
Once the image is captured, the technologist makes a visual assessment of the image quality, typically on a low-resolution, low-dynamic range monitor, and may also refer to the EI to determine if the image was properly exposed. The exposure indicator is a FIGURE of merit that is calculated for the captured image and that is related to the average signal level for the anatomical region of interest, prior to post-processing.
The technologist may decide to repeat the image if the EI is too low, or if the image appears noisy, i.e., if the image is deemed to be underexposed, or alternatively, may choose to modify the post-processing.
To reduce the number of images that may need to be repeated because of underexposure, and because there is some variability associated with the choice of exposure parameters, it is a typical practice to set the exposure parameters well-above the minimum level that is required to produce a diagnostic quality image. Consequently, patients that are imaged using standard x-ray machines may often receive a considerably higher radiation dose than that which is required for diagnosis. This can be a particular problem in intensive care units, where patients typically receive one or more chest x-rays per day, including pediatric and neonatal intensive care unit patients.
Hue-saturation-luminance models (HSL) typically map colors onto a three-dimensional prism-map comprising two symmetric cones stacked face-to-face. A given color may be thus described using cylindrical coordinates on the prism: the central vertical axis of the prism represents the luminance channel, the angle around the central vertical axis represents the hue, or chroma channel, and the distance from the central vertical axis to the surface of the prism represents the saturation channel Due to the conus-shape at either end of the prism, the range of the saturation channel depends on the luminance: in the middle of the range, the saturation channel provides a wide range of levels, whereas at the extrema comprising very dark or very light luminance values, the saturation channel provides a narrow range of levels. Additionally, human perception of different hues is not uniform—some hues, such as red hues, are perceptible over a wide range saturation levels, whereas other hues, such as blue hues, are perceptible over a narrower range of saturation levels. Thus, the surface of the HSL prism-map is not consistently convex, but includes bumps and dents.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the figures.